Alternating-current signal transmitting system



May 24, 1949. H, GARDERE 2,471,319

ALTERNATING CURRENT SIGNAL TRANSMITTING SYSTEM Filed May 2o, 1946 2 sheets-sheet 1 May 24, 1949. H. GARDERE ALTERNATING CURRENT SIGNAL TRANSMITTING SYSTEM 2 sheets-sheet 2 Filed May 20, 1946 IDC. SIGNILS5 SIGNAL. Ampmmsa is BPF 7 AL. SIGNALS (PHASE MoouLArso) -(J FI G 4 ADENTODULA'TQR l N VENTO@ HE/VE/ 3Q/Posee Patented May 24, 1949 ALTERNATING-CURRENT SIGNAL TRANSMITTING SYSTEM Henri ardre, Paris, France, assigner to Compagme Generale dElectricite, Paris, France, a corporation of France Application May 20, 1946, Serial No. 670,891 In France May 7, 1945 Section 1, PublicLaw 690, August 8, 1946 Patent expires May 7, 1965 4 Claims.

The present invention relates to the transmission of telegraph code signals and has for its object to provide a frequency change signal transmitting system of the so-called double current and phase modulation type, characterised by the fact that: at the transmitting end the direct current signals of both positive and negative types are converted into two types of alternating current signals having a phase difference of between 0 and 1r; at the receiving end the carrier current is reconstituted from the signals with a constant phase, and by means of said carrier current the alternating current signals are demodulated (i. e. reconverted into direct current) without any uncertainty as to their correct polarity and having durations which are independent of the amplitude of the alternating current signals. In a double current alternating current system, there is transmission of alternating current during both the spacing keying intervals and the marking keying intervals.

The advantages of such double current telesignal systems are known, such advantages being due to the fact that the signals reconstituted at the receiving end are independent of the amplitude of the changed frequency current signals changed at the input of the receiver. The known double current telesignalsystem operating by means of reversals of the phase of the alternating current in synchronism with the reversals separating the direct current signals has this advantage, but does not reliably give the signals with their correct polarity at the -beginning of a communication. The receiver cannot determine the polarity of the direct current signals which correspond to the alternating current signals received, since the successive signals comprise reversals which indicate nothing regarding the absolute value of the polarity of the signals. Only the relative polarity of the signals is known, i. e. that a signal is of opposite polarity to the previous or to the following signal.

From this it is apparent that in these known double current systems it is only possible to determine the absolute polarity by conventions relating to the actual signal code. Thus electromechanical analysers have been constructed which enable the exact absolute polarity to be recognised by utilising peculiarities of the signal code used. But in addition to the fact that this device complicates reception, it is only applicable to the onesig'nal system, that is, a system using a determined type of signalingsuch as Baudo-t, Wheatstone, Hughes, or start-stop.V

It isl due to` this difculty that double current transmission involving frequency change (and carrier current) have not heretofore been as generally used as should have been warranted by their property of independence of amplitude at the receiving end. Single current communications remain the most generally used despite their distortion which is function of the reception level and, in order to minimize this serious defect, there is at present a tendency to allot to them a. wider frequency band, which makes them less economical in spectrum requirement.

Fig. 1 shows one form of receiving circuit for receiving doubly phased modulated carrier signal currents and converting them into direct current signals, using a balanced demodulator, the pclarity of the initial direct current element corresponding to the polarity of the initial element of the signal applied to the transmitter;

Fig. 2 shows the vector relations in the receiving circuit of Fig. 1, when the first element of the received signal is a spacing element;

Fig. 3 shows the vector relations in the receiving circuit of Fig. 1, when the first element of the received signal is a marking element;

Fig. 4 shows a modified form of circuit using a single demodulator for'accomplishing the same result as the arrangement of Fig. 1;

Fig. 5 shows the vector relations in the receiving circuit of Fig. 4, when the rst element of the received signal i9 is a spacing element; and

Fig. 6 shows the vector relations in the receiving circuit of Fig. 4 when the rst element of the received signal is amarking element.

In the system according to the present invention, the signal changer at the transmission end produces alternating current signals which do not differ by a change of sign, but by a change of phase which is smaller than 1r in absolute value. For instance, the alternating current corresponding to a negative direct current marking signal as produced by the key, has a phase advance of an angle qs which is smaller than 1r with respect to the alternating current corresponding to a space signal. Obviously it would be possible to make the reverse convention, viz. a mark signal having a phase lag of qs with respect to the space signal.

In the known double current system of the prior art referred to above, operating by reversals of the alternating current, the direct current signals which are reconstituted at the receiving end are not dependent on the amplitude at the input of the demodulating receiving system, because the duration of such reconstituted signals is equal to the time between the reversals of the alternating current signals. In the system according to the present invention, the same advantage is obtained, because the duration of the reconstituted signals is equal to the time between the changes of phase of the alternating current signals and is not dependent on their amplitude. But whereas in the prior art double current system operating by reversals, the absolute polarity of the signals cannot be determined, in the system according to the present invention, it is clear, as soon as the second signal appears in the receiving demodulator, whether said signal is a mark signal or a space signal- (i. e. negative or positive), according to Whether said signal has l an advance or a lag of the phase angle in accordance with the convention hereinbefore agreed. Thus the reconstituted signals have the same polarity as at the transmitting end, with means independent of the code of the signal or telegraph system used;

The change of phase. at the transmitting end can be obtained, according to the present invention, in the following manner: vtwo sources of alternating current of the same frequency and withfa phase difference equal to gb alternately transmit their current to the line through one or a plurality of dynamicI or static, electromechanical or electronic relays, which are controlled by the direct current signals of the signal or telegraph apparatus. In accordance with the convention hereinbefore agreed (alternating current having a phase advance of for a direct current mark or negative signal with respect to the alternating current for a` spacing signal element as produced by the key), the mark signal or negative direct currentA operates the relay or relays which connect to the line the source of alternating current having such a phase advance of vqb. Said vsource is replaced by the other alternating current source when the relay or relays are operated by a direct current spacing or positive signal.

Obviously the two sources could be replaced by a single source provided with two parallel leads connected to one or two dephasing devices producing at the output of said leads currents which are 4; out of phase, andl instead of connections being made tothe two sources, the co-nnections are made to the two parallel leads.

Another example of frequency change means producing signals which are out of phase by consists in permanently connecting the alternating current source to the line and inserting a dephasing device between the source and the line by means of relays controlled by the direct current signals. It is thus possible to obtain, applying one of the two (positive or negative) types of direct current signals, the phase difference of e in question.

Another example, according to the present invention, of frequency change at the transmitting end, that is, the transformation of D. C. signals (zero frequency) into A. C. signals of the carrier frequency, consists in using a known frequency changer of the double current type as employed in the prior art operating by reversals of the alternating current, and in transmitting to the line, in addition to the double current alternating current signals produced by reversals, an auxiliary current which has a mld 4 phase advance with respect to one, and a phase lag with respect to `the other, of the two alternating current signals. If it is desired to have a phase lag of .qb between the two resulting signals, A being the amplitude of the signals avt the output terminals of the frequency changer, and a the amplitude of the auxiliary current, the following equationk must be satisfied:

The auxiliary current must have an advance with respect to the space signal current, if adhering to the aforesaid assumed convention of a mark signal current having an advance of phase with respect to the space signal current. Another example consists in using a variable phase source of alternating current and controlling the phase by. means of a relay controlled by the direct current keying signals.

The signals thus obtained from the transmit.- ter reach the input of the demodulating device at the receiving end in the form of alternating current. According to the aforesaid convention, it is assumed that the alternating current corresponding to a mark signal has a phase advance. of 4; with respect to the alternating current of the same frequency corresponding to a space signal. It is obvious that the converse convention could be made, but in order to simplify the explanation the rst mentioned convention will be adhered to throughout the remainder of the present specification.

Moreover, in order to simplify further, it will be assumed that the phase diierence has the value Fig. 1 shows the diagram of a system according to the invention adapted to convert alternating current signals into direct current signals at the receiving end; the input terminals to which the alternating current signals is are fed are located at e. fs is a preliminary filter for the signals which is particularly intended to separate the signals of various communications on several different frequencies, in the case of multiplex channels as known. For the purposes of' the transmission, said preliminary `iilter passes a band of at least cycles, t being the elementary time,.i. e. the duration of the shortest possible interval; the maximum band width is determined by the illtering requirements or by the desirability of eliminating or attenuating parasites.. The filter maybe simple, this being an advantage involved in double current alternating current communication, as the filtering can be completed after demodulation, simply by a low-pass filter.

At the output terminals of the fs lter, d1 is a parallel lead through which the signal current can reach either the input terminals of the signal amplier as, or the input terminals of the blocking devices br, bt. These blocking devices are units which only allow signal current from di to pass from the input terminals to the output terminals (from the left-hand terminals to the right-hand terminals) when they are supplied with direct current at their control terminals (upper terminals). Such blocking units are well known in weak current technique and may consist of a simple network of rectiers.

'I'he output terminals of the blocking devices br and bt are in parallel, i. e. connected to one another. But Whereas the connecting leads are directly connected to the output terminals of br, the output terminals of bt are connected, ahead of the leads, to a dephasing device st which produces a lag of in the currents that pass through it. The output leads are connected to the input terminals of the carrier current band pass iilter fp. Said lter, which is centered on the carrier frequency, should pass a band width of less than t being the duration of the elementary signal. But more often it will be an advantage to have as reduced a band width as possible compatible with the stability of the elements and of the carrier frequency. The output of the carrier filter fp is fed direct to an amplier ar for the space signal carrier current and, through a dephasing device so producing a lag of to an amplifier at for the mark signal carrier current.

The aforesaid signal amplier as is connected to the input terminals of two demodulators in parallel, the space signal current demodulator mr and the mark signal current demodulator mt. The output terminals of the demodulators (righthand terminals) are also connected in parallel.

The control terminals (upper terminals) of the demodulators mr and mt are respectively connected to the output terminals of the ampliers ar and at. The demodulators are units of the known balanced type called carrier current suppressors. They are such that if a current i(t) is applied to the input terminals and a current I(t) to the control terminals, they produce at the output terminals a current of the form a.z`(t).I(t), a being a constant. It ensues from this property, as is well known, that if currents of the same frequency but out of phase by an angle Il are applied to the input and to the control terminals, a direct current proportional to cos c is obtained at the output terminals which is, therefore, independent of the amplitude of the input.

The output terminals of the demodulators are connected in parallel and are connected to a low-pass lter fc, the cut-olf frequency of which is at least equal to lgt, t being the duration of the shortest possible signal. A control circuit dal is connected across the control terminals of the blocking devices br and bt through valves. or rectiers rr and rt which only allow current to pass in the direction of the arrows that represent them. This control circuit d2 is connected in series in one of the leads connecting low pass lter fc to the output terminals of demodulators m1' and mt.

Fig. 1 enables an explanation to be given of the operation of the system illustrated therein by way of a non-limitative example, according to the present invention, of the receiving portion which converts the alternating current signals identical to those transmitted.

Two cases may arise: either the rst signal is a space signal, or the iirst signal is a mark signal.

l. First casa-Assume that when the transmission is started the iirst alternating current signal that reaches the input e of the receiver of Fig. 1 is a space signal current. It passes through the band-pass lter fs-which may be a simple one (the system enabling the signals which have been re-converted into direct current signals to be ltered by a simple low-pass iilter then the current reaches the parallel connection di. One'portion of the current is amplified in the signal amplier as and is fed to the input terminals of the two demodulators in parallel mr and mt (lefthand terminals of the demodulators). The other portion of the signal which is shunted at di passes through the blocking device br.

It was stated hereinbefore that the blocking devices br and bt only allow current to pass between the input and output terminals when their control terminals are supplied with direct current, therefore if the blocking of br and bt is complete, the current is a zero current, save the initial signal current; in the initial state br has to be blocked.

In order that the signal current shall be able to pass through b1', the same is so designed that in the blocked state it allows a current to pass which is small compared with the current that would pass with br open. The current having passed through br, flows into the carrier current filter fp. At the output of the carrier current filter fp. the signal current which is still of the same phase (space) is shunted towards the input terminals of two amplifiers: directly across the input terminals of the space signal current amplifier ar and, through a dephasing device sp which throws it These two amplifiers respectively supply the control or carrier current terminals of the two demodulators mr and mt which, moreover, as hereinbefore stated, are fed in parallel through their input terminals by the signal current amplied by signal amplifier as. At this moment,

the demodulator mr which is fed through its in-4 put terminals and through its carrier current control terminals with currents which are in phase, supplies a direct current across its output terminals; the connections are in a direction such 7 that said direct current flows through the lowpass filter ,fc and the output terminals s in the direction of the arrow which is assumed to be the positive direction. The demodulator mt which is fedv with currents which are Lol s the valve rr and the blocking device br which is thus made open, whereas the blocking device bt, through which no current passes owing to the direction of the valve rt, remains blocked. The blocking device br therefore allows the current derived at d1 to pass freely towards the carrier band pass filter fp, which only increases the weak current that it allowed to pass at the outset. This also increases the currents in the various circuits with their polarity and their phase as they were hereinbefore described.

When the received initial space signal applied at e is replaced by a marl: signal, the flow of this current involves (according to the assumed convention) a phase advance of in the alternating current forming the signals. Owing to the high time constant of the carrier current lter fp, the phase and the amplitude oi the current at the output terminals of said lter fp can only change with a certain delay, so that the carrier currents amplified by amplifiers ar and at have not changed when the mark signal,

which has a phasevadvance of is applied to the input terminals of its demodulators.

The demodulator ar which produced a positive current, now at this instant produces no current since the current at the input terminals and the carrier current are out of phase. The demodulator mi, which produced no current, is still supplied with a carrier current which has a phase lag of and now has at its input terminals a signal current which has a phase advance of but it passes through the blocking device bt which owing to this fact is open, i. e. has only a small attenuating eiiect on the signal current flowing from the parallel leads di. Instead of, as in the previous case (space signal current), the current (marking signal) passing through the blocking device br and reaching the carrier current lter fp, it now passes through the blocking device bt and reaches the lter fp through the dephasing device st which imparts to it a phase lag of Now, in the present case the current representing a mark sign-al which, with respect to the previous signal, has a phase advance, next after passing through the dephasing device st, gets a phase lag, thus undergoes two successive equal and opposite changes of phase, the overall eect of which is nil. Therefore the current at the input terminals of the filter fp does not undergo change of phase.

At the instant when the space signal current changes to mark signal current, there may ci course be a short instant during which the current at the input terminals of the lter fp changes its amplitude or its phase, but such instant is short compared to the time constant of the iilter, so that if the phase remains constant at the input terminals of the iilter fp for greater part of the time, it may be considered to be absolutely constant at the output terminals of the iilter for the purposes of demodulation.

The analysis of the operation gives, for the time constant of the filter fp, a value at least equal to one half of the duration of an elementary signal (i. e. of the shortest possible signal) for the current at the output terminals to remain sumciently constant. Thus the above explanation shows that if the first signal that arrives is a space signal, the system described produces a positive direct current for a space signal and a negative direct current for a mark signal. Before reaching the output terminals s, the signals which have been re-converted into direct current pass through the low-pass lter fc which eliminates the unwanted demodulation components and the components of adjacent signaling channels which might remain.

2.. Second case-In the other contingency when the iirst signal that arrives is a mark signal, it is obvious that the system described cannot detect its phase in an absolute manner. For this first signal the receiver produces, as in the iirst case, a positive direct current which is consequently incorrect. The system operates as previously described, for any initial signal whatever be the phase of such signal. The operation changes as soon as the second signal appears, which will now be a space signal. The arrival of this second signal produces (change from marl: to space) a phase lag of rola During the first signal (as in the rst case) the demodulator mi', whose input terminals were supplied with the signal current and the control terminals with a carrier current of the same phase, produced a positive current at the output terminals, whereas the demodulator mt, which 9 was controlled by a carrier current having a phase lag of produced no current. The positive current at the output terminals of the modulator mr opened the blocking device br.

At the beginning of the second signal which has a phase lag of with respect to the previous mark signal, the demodulator m1* produces no demodulated current, and the demodulator mt (still supplied with carrier current having a phase lag of with respect to the rst signal.

At the output terminals of the carrier filter fp and throughout the duration of the second signal,

the phase of the carrier current gradually changes, according to the time constant, from the phase of the rst signal to the phase of the second. Simultaneously, the two carrier currents which supply modulators mr and mt, undergo the same change of phase (backward displacement of and the current z'mr demodulated by mr gradually changes from 0 to its normal positive value, whereas the current imt demodulated by mt gradually changes from its positive value to 0.

Throughout all this change, the two demodulators mr and mt as a whole, produce a positive total current and the blocking device br remains open. After the change of phase of the carrier currents imr and zmt, the space signal is demodulated by the demodulator mi', whereas mt produces no current. The system has thus returned to the state in which it was in the rst case when a rst space signal was being demodulated. Thus, apart from the first signal which may be incorrectly received if it is a mark signal, the system produces a positive current for the space signals and a negative current for the mark signals.

In order to give greater clarity to the description of the foregoing operations, of the circuit of Fig. l, there is shown for the first case when the first signal received is a spacing signal, in Fig. 2: at a the vectors z'sr representing the space signals and ist with an advance of representing the mark signals; at b and at c the signals and currents illustrating the first case of operation. At b, zsr represents the initial space signal applied to the system, i101 the carrier cur- 10 rent produced by the first signal and fed to the demodulator mr, pt which has a phase lag of representing the carrier current fed to the demodulator mt, zmr is the positive current produced from z'sr and z'pr in phase in the demodulator m1' and z'mt is the zero current demodulated from is? and z'pt in quadrature in the demodulator mi.

At c, ist represents the mark signal following the initial signal, pr and zpt the carrier currents which remain permanently owing to the effect of the large time constant filter fp and of the opening of the blocking device bt instead of br, z'mr is the zero current produced from ist and z'pr in phase quadrature in m1', and zmt is the negative current produced from ist and ipt in phase opposition.

In Fig. 3 have been shown for the circuit of Fig. 1 the signals and currents illustrating the second case of operation when the rst signal that arrives is a marking signal. At d in Fig. 3, ist is the initial mark signal, z'pr and pt the carrier currents produced thereby and having a phase advance with respect to their normal position, mr the positive current produced from ist and zpr in phase in mr, and z'mt the zero current produced from ist and zpt in phase quadrature in mt. At instant f (Fig. 3), z'sr is the space signal current that follows the current of the initial signal, ipr and z'pt the carrier currents which have not yet had the time to change and are identical with those shown at d (Fig. 3). imr is now zero and z'mt produced from z'sr and z'pt in phase in mt is positive. At instant g, h and z' is shown the backward rotation of the carrier currents ipr and zpt during .the space signal zsr which has a phase lag with respect' to the previous signal. In the lower row of Fig. 3 are shown the corresponding variations ofthe demodulated currents produced. z'mr which is Zero at instant acquires increasing positive values and imt retains positive but decreasing values. At instant y is again the space signal z'sr, the carrier currents have reached their normal phase, z'mr is positive and zmt is zero. The system is in the state shown in Fig. 2 at d and is therefore ready to receive correctly the next mark signal.

Fig. Il shows the diagram of another nonlimitative example, according to the present invention, of a system which re-converts at the receiving end the phase modulated alternating current signals into direct current signals. At e are the receiver input terminals, fs is a prefiltering filter having the same functions and the same characteristics as in the previous example of Fig. l, d1 is a parallel connection through which the current of the signals from js can reach, either the input terminals of the signal amplifier as, or the input terminals of the blocking devices br and bt in parallel. Said blocking devices are units comprising rectier bridges which produce between their input and output terminals (extreme left-hand terminals for the input, extreme right-hand terminals for the output) a small or large decrease of intensity according to whether direct current owing in one direction or in the 11 other is fed to their control terminals (mid terminals on both sides.)

Thus blocking devices br and bt produce a negligible decrease when direct current is passing through them in the direction of the arrow (arrow and at the output terminals of the blocking device bt another dephasing device st imparts a phaselag of (still assuming the convention of a mark signal current having a phase advance of with respect to the space signal current). Following the two dephasing devices, the circuits fed with the output of the two blocking devices are connected to the input terminals of the bandpass lter fp.

This band pass lter fp which is centered on the carrier frequency has similar characteristics to those ci the filter fp o the previous example of Fig. 1. The output terminals of the, lter fp are directly connected to the input terminals of the carrier current amplier ap. The output terminals oi the signal amplifier as previously referred to are connected to the input terminals (left-hand terminals) of the demodulator m, the f'' control terminals of the demodulator m (upper terminals) are connected to the output terminals of the amplifier ap. The demodulator m is a unit of the typeY of those used in the previous example of Fig. 1. The output terminals of the demodulator are connected to. a low-pass lter fc having similar characteristics to those of the filter of the same designation in the previous example of Fig. 1. Between the output terminals of the demodulator m and the input terminals of the lter fc, a parallel lead d2 is connected to the control terminals of the blocking devices br and bt. In the ensuing explanation of the operation of this second modification, reference will be made to Fig. 4 hereinbefore described.

As in the previous example of Fig. 1, two cases have to be considered according to whether at the outset the first signal to reach the receiver is a space signal or a mark signali 1. First casa-The first signal is a. space signal. The current of this signal passes through the lter fs, reaches the parallel lead di. One portion of said current is amplifled in the signal amplifier as and is fed to the input terminals of the demodulator m. The other portion of the signal which is shunted through di flows along two parallel leads and partly passes through the blocking devices br and bt which, owing to the initial absence of direct. current at their control terminals (mid terminals) are neither open nor lli Gil

blocked. The signal current then passes through the dephasing devices s1' and st, reaches the input terminals of the carrier band pass filter fp, passes through same and is then amplied by the carrier current amplier ap and fed to the control terminals of demodulator m.

The blocking devices br and bt should be so designed that their transmission properties, in the absence of direct control current, allow such currents to pass that at the output terminals of ap and at the control terminals of m, there is a current which diiers in phase by less than from the initial signal current at the input terminals of the demodulator m.

Thus the demodulator m, to the input and control terminals of which are fed currents that differ in phase by less than produces at its output terminals a positive direct current im (direction of the arrow at the output terminals of m). Said direct current, which is diverted at d2, passes through the two blocking devices br and bt by way of their control terminals and, owing to its polarity, opens br and blocks bt. The signal current from the shunt lead di which was passing through the blocking devices br and ht now only passes through br, is given a phase lag of by the dephasing device sr, passes through fp and, after being amplified by ap, is fed to the control terminals of m, The demodulator m,

, which now has currents differing in phase by fed to its input and control terminals, produces a positive current im at its output terminals. Thus the initial current im will be increased in the same direction so long as the initial space signal current lasts, producing at the output terminals after passing through fc a space signal correctly re-converted into direct current.

When the initial space signal is replaced by a mark signal current, the flow of this current involves (according to the assumed convention) a phase advance of Owing to the large time constant of the carrier current filter fp, the phase and the amplitude of the current at the output terminals of said filter can only change after a certain delay, so that the carrier current amplified by amplier ap and fed to the demodulator m has not changed (still has a phase lag of when the mark signal is applied to the input terminals of the demodulator. The mark signal has a phase advance of with respect to the initial space signal, the carrier current a phase lag of 13 This mark signal current and the carrier current are therefore out oi' phase by:

1r 1r 31| rift-"4 i. e. more than and the demodulator will produce a negative current at the output terminals. This negative current through the parallel lead d2 closes br and opens bt. The mark signal current from d1 having a phase advance of passes through bt, is given a phase lag of by the dephasing device st and reaches fp with a phase lag of Thus the current at the input terminals of the carrier band pass filter fp does not change its phase.

At the instant when the space signal changes into a mark signal, there may possibly be a short instant when the current at the input terminals of fp changes its amplitude or its phase, but this instant is short compared to the time constant of the filter, so that the carrier current may be considered to be of constant amplitude and phase at the output terminals of the iilter fp and therefore at the control terminals of the demodulator. The demodulated current im will therefore be negative as long as the mark signal current lasts. The space and mark signals are thus reconstituted with their correct polarity.

2. Second case-The iirst signal is a mark signal. It is obvious that at the beginning of a message, that is to say as soon as an alternating current flows at the receiving end, the system cannot indicate the phase of such current, since a current only has a phase relatively to another current or relatively to a denite origin of time. For this iirst mark signal, the system this will therefore behave exactly as in the first case,

i. e. it will produce at the output terminals a positive and consequently incorrect current. There will therefore be, at the control terminals of the demodulator m throughout the whole duration of the rst signal, as in the previous case, a carrier current which has a. phase lag of with respect to the signal current. Similarly, the positive demodulated current im will open the blocking device br and close ht. But at the arrival of the second signal (space signal) which now has a phase lag of with respect to the previous signal, the difference of phase between the carrier and signal currents at the terminals of m changes from 1r. minus 4 to plus this difference remaining smaller than so that the demodulated direct current remains positive.

'I'herefore the blocking device br will again remain open and the signal current at the input terminals of the filter fp will have a phase lag of (relatively to What it was during the rst sig- I- nal). The output current of the filter fp, and

the current at the output terminals of carrier amplifier ap and at the control terminals of the demodulator m will therefore gradually change its phase according to the time constant of the carrier band pass lter fp. Like the change of phase of the signal, this change will be a lag of rola but slower, the phase of the carrier current relatively to the signal current will therefore change from a advance to a lag. It follows that during any change of phase of the carrier, the demodulated direct current will remain positive.

'I'he second signal (space signal) is therefore correctly received and the system is in the conditions of the rst case (rst signal a space signal) after the reception of the first signal to receive correctly the next mark signal. Thel filter fc through which the demodulated currents pass before reaching the output terminals, enables the unwanted components of demodulation to be eliminated and also the components of adjacent signal systems on other frequencies, which the lter fs may have allowed to pass.

In order to give greater clarity to the fore-v going description of operation of the circuit of Fig. 4, there is shown in Fig. 5, for the rst case when the first signa-l received is a spacing signal, the vectors representing the currents. At lc are shown the two vectors zsr rep-, resenting the alternating current space signals and ist with a phase lag of im is the positive demodulated current produced from isr and ip which are given a phase dierence of in the demodulator m. At n, ist represents the mark signal current, following the initial signal, ip the carrier current which does Vnot un- 15 dergo a change of phase and therefore has a' phase lag of with respect to ist owing to the eiect of the time constant of the filter fp, of the opening off the blocking device bt and of the dephasing device st. im is the negative direct current produced from ist and ip which are given a phase lag of alga in the demodulator m.

In Fig. 6 have been shown for the circuit of Fig. 4, the signals and currents illustrating the second case of operation when the rst `signal element received is a marking signal. At p, ist is the initial mark signal, ip the carrier current which results therefrom and has a phase lag of as in the rst case. im is the positive and therefore, incorrect direct current produced from ist and ip in the demodulator m. At instant q, isfl is the space signal following the initial signale and having a phase lag cf with respect to same, ip is the carrier current which has not had time to change, an im the demodulated current resulting therefrom and which is always positive. At instants 1', u and o can be seen the gradual backward rotation of the vector representing the carrier current 'ip throughout the duration of the signal isr. In the bottom row of Fig. 6 Vis shown the resulting demodulated direct current im. It is always positive, at instant r it increases, at instant u it1 passes through a maximum value, at instant o it decreases. Finally, at instant a: there is always the space signal ier, the carrier current ip has reached its normal phase, im is positive. The system is in the condition shown in Fig. at l, therefore ready to receive correctly the coming mark signal.

It has not been mentioned in the previous eX- arnples, but it is an advantage of double current signal systems and of the present invention that it is possible to add to the signal amplifiers as an amplitude and level regulator Which is simple and easy to construct owing to the fact that all the signals involve a flow of current. This latter point is in contra-distinction to single current signal systems which have zero current signals, that is, there are intervals when no current is transmitted, between intervals of different lengths when current is transmitted. It is thus possible to make the carrier current constant, i. e. independent of the slew variations of amplitude of the signals, by means of a simple limiter adjusted to its minimum amplitude. Said limiter is preferably located after the filter fp or is added to the ampliiiers ai', at (Fig. l) cr ap (Fig. 4).

The system according to the present invention offers the advantage of giving unequivocally the polarity of the re-converted signals whose distortion is practically independent of the strength of reception. The signals are re-converted according to their type in an exact and well-dened direction. This property involves the possibility, without impairing the transmission, of narrowing the frequency band allotted to each communication down to the requisite theoretical value. Thus the usual single current communications, at the signal speed of 59 bauds, are allotted a frequency band of to 90 cycles with a serious distortion when the strength of reception varies, and owing to this defect there is a tendency to increase this band width to a value er;- ceeding cycles. The system according to the present invention enables the same message to be transmitted, with a frequency band of only 50 cycles per second, therefore in a multiplex system, two or three times more messages to be transmitted, with better quality of reconstitution.

Furthermore, the system according to the present invention oiers the advantage, which is common to all double current signal systems, of being less sensitive to parasites; it requires during transmission smaller power than that used for single current signal systems, and thus has less effect on neighbouring communication.

The system according to the present invention is applicable to all signal systems in which it is an advantage to use alternating current rather than direct current signal, in particular in simple or multiplex signal or tele-communication systems, either wired or wireless. The system can be applied equally well whatever be the transmitting medium and agent, for example: wire and electric current, space and Hertzlan or light waves, atmosphere and sound waves, and so forth, provided that at the transmitting end there is conversion of the electric current of the signals according to the present invention into the agent utilized, and the reverse conversion at the receiving end. In particular, in communications by means of Hertzian waves, the systems described is to a great extent insensitive to fading.

I claim:

1. In a receiver for receiving and reconverting into direct current signal elements ci successively opposite polarity, a transmitted alterhating current signal consisting of successive short trains of two alternating currents of the same frequency but having a substantial determined phase diierence W which is substantially less than 1r, receiving terminals for receiving said trains of the so determined different phase, a carrier current suppressing balanced demo-dulator unit having input terminals and output terminals and control terminals, said input terminals of said demodulator unit being connected to said receiving terminals, a pair of blocking devices having substantial conducting asymmetry and each having input terminals and output terminals and control terminals and each being adapted to pass a relatively large current from its input terminals to its output terminals when rect current of a particular characteristic is applied to its control terminals and to pass only a relatively small current from its input terminals to its output terminals when its control terminals are not so energized, a carrier band pass lter of relatively large time constant, a connection between the input of said carrier filter and the output terminals of said blocking devices, said connection comprising dephasing means for dephasing the output of one cf said blocking devices relative to the output of the other blocking device by a substantial angle which is substantially less than 1r and 4equal to said determined phase diierence W oi said two alternating currents appli-ed to said receiving terminals, the output of said carrier ilter being connected to the control terminals of said demodulator unit, direct current output connections for-'saidreceiver yconnected to the output terminals of said demodulator unitpand control connections from the output terminals of said demodulator unit to the control terminals of said blocking devices, said control connections and said blocking devices comprising means whereby one and only one of said blocking devices at a time is selectiv-ely unblocked to pass a relatively large current according to the polarity of the direct current output of said demodulator unit, whereby the output of said demodulator unit consists of successive direct current signal elements of both polarities as uniquely determined by the phase of the received alternating current signal trains and whereby the phase of the currents applied to the input of said carrier filter is substantially constant and the amplitude of said output is independent of the amplitude of said transmitted alternating current signal trains.

2. In a receiver for receiving and reconverting into direct current signal elements of successively opposite polarity a transmitted alternating current signal consisting of successive short trains of two alternating currents of the same frequency but having a substantial determined phase difference W which is substantially less than 1r, receiving terminals for receiving said trains of the so determined diierent phase, a pair of carrier current suppressing balanced demodulators each having input terminals and output terminals and control terminals, said input terminals of said demodulators being connected to said receiving terminals, a pair of blocking devices having substantial conducting asymmetry and each having input terminals and output terminals and control terminals and each being adapted to pass a relatively large current from its input terminals to its output terminals when direct current of a particular characteristic is supplied to its control terminals and to pass only a relatively small current from its input terminals to its output terminals when its control terminals are not so energized, a carrier band pass lter of relatively large time constant, a rst connection between the input of said carrier filter and the output terminals of said blocking devices, said i'lrst connection comprising i'lrst depliasing means inserted in the connection to the output terminals of at least one of said blocking devices for dephasing the output of one of said blocking devices relative to the output of the other blocking device by a substantial angle which is substantially less than 1r, and equal to said determined phase difference of said two alternating currents applied to said receiving terminals, second connections between the output of said carrier filter and control terminals of said demodulators, said second connections comprising second dephasing means inserted in the lead to the control terminals of at least one said demodulator for dephasing the control current applied to the control terminals of one said demodulator with reference to the control current applied to the control terminals of the other said demodulator by a substantial determined constant phase diierence angle which is substantially less than vr direct current output connections for said receiver connected in parallel to the output connections of said demodulators, and control connections from the output terminals in parallel of said demodulators to the control terminals in parallel oi said blocking devices, the lead of said control connections to the control terminals of a first one of said blocking devices comprising a, rst asymmetrical conductor element, and the lead of said-control connections to the control terminals of the other said blocking device comprising a second asymmetrical conductor element which is poled oppositely to said first conductor element, said control connections and said asymmetrical conductor elements being soarranged that one and only one of said blocking devices at a time is selectively unblocked to pass a relatively large current according to the polarity of the direct current output of said demodulators, whereby the output of said demodulators consists of successive direct current signal elements of both polarities as determined by the phase of the received alternating current signal trains and the phase of the current applied to the input of said carrier filter is substantially conetant and the amplitude of said output of said demodulators is independent of the amplitude of said received alternating current signal trains.

3. A receiver according to claim 2, said determined phase diierence of said two alternating currents applied to said receiving terminals and of said first and second dephasing means, being substantially equal to 4. In a receiver for receiving and reconverting into direct current signal elements of successively opposite polarity, a transmitted alternating current signal consisting of successive short trains of two alternating currents of the same frequency but having a determined phase difference which is substantially receiving terminals for receiving said trains of Y the so determined different phase, a carrier current suppressing demodulator having input terminals and output terminals and control terminais, said input terminals of said demodulator being connected to said receiving terminals, a pair of blocking devices having substantial conducting asymmetry and each having input terminals and output terminalsv and control terminals, and each being adapted to pass a relatively large current from its input terminals to its output terminals when direct current of a particular polarity is applied to its control terminals and to pass only a relatively small current from its input terminals to its output terminals when direct current of opposite polarity is applied to its control terminals or when its control terminals are not energized, said two blocking devices being poled to pass relatively large currents upon application of opposite polarities to their respective control terminals, a carrier band pass filter of relatively large time constant, connections between the input of said carrier iilter and the output terminals of said blocking devices, the connection from a rst one of said blocking devices to said carrier filter comprising first dephasing means adapted to introduce a phase displacement of between its input and its output, the connection from a second one of said blocking devices to said carrier filter comprising second dephasing means adapted to introduce a phase displacement of I9 between its inputand its' outputf theoutput of said carrier filtei beingconnected to the control teiniinals of said deinodula-tin',l direct current oiitpiit yctuiiections for saidreceiver connected to the output terminals of said demodulator, and control connections from the output terminals of saiddemodul'atox" to the control terminals of said blocking devices, 'said control connections and thepoling of said blocking devices being such that one and onlyv one of saidhlocking devices at a time is selectively unblocked to. pass a relative- 1y large current according' to the polarity of the direct current output of said demodulator; Where-a by theyo'utput -of said deinodulator consists of Success-ite direct Acurrent signal elements of both polarities as uniquely determined by the phase of the received'alternating current signal trains and thepliase of ,the current applied to the input of said carrie iilter is substantially constant and 20 the amplitude of said output is independent of the amplitude of said alternating current signal trains.

HENRI GAR/DEBE.

REFERENCES CITED The feuowing references are of record in the fue of this' patent: 

